1. Field of the Invention
The present invention relates to an apparatus and a method for decoding an AM stereo broadcasting signal of an independent sideband system and particularly to an apparatus and a method for reducing distortion of a stereo difference signal at the time of decoding an AM stereo broadcasting signal of the independent sideband system.
2. Description of the Prior Art
Several proposals have been made concerning an AM stereo broadcasting system. One of them is an independent sideband system (referred to hereinafter as ISB system) in which a left stereo signal L and a right stereo signal R are carried onto an upper sideband and a lower sideband, respectively, of a carrier, disclosed for example in U.S. patent Ser. No. 4,018,994. According to the ISB AM stereo broadcasting system of this document, a carrier is subjected to amplitude modulation by a sum signal (L+R) phase-shifted by +45.degree. and is also subjected to phase modulation by a difference signal (L-R) phase-shifted by -45.degree., whereby the left stereo signal L and the right stereo signal R are modulated to the lower sideband and the upper sideband, respectively. However, only by modulation of the carrier, a vector formed by phase modulation does not have always 90.degree. with respect to the carrier and an error is caused, resulting in distortion of a signal at the time of decoding thereof.
As a method for solving such problem, a reverse modulation system is proposed for example in U.S. patent application No. 912,281 filed May 5, 1978.
However, the reverse modulation system involves a problem that a circuit for reverse modulation of a carrier has a more complex configuration. In addition, there is involved a further problem that in order to precisely compensate for distortion of a signal, it is necessary to set a reverse modulation function with an extremely high precision, making precise control of the circuit necessary.